In accordance with the general terminology utilized in the printing industry, the word "letterpress" refers to a printing procedure in which the locations on the plate where ink is to be deposited are raised with respect to areas where ink is not to be deposited. Within the general designation of letterpress printing, two distinguishable forms can be identified. The first typically utilizes a relatively stiff plate (i.e. employing a material of relatively high stiffness), commonly referred to in the industry as a "hard" plate. "Hard" plate letterpress systems typically employ an impression roll with a compliant coating and one or more form cylinder(s) also with compliant coating(s). The form cylinder may be either directly inked from a well, or remotely inked through a series of rollers. The ink on the form cylinder is transferred to the inking locations on the "hard" plate which is mounted to the plate cylinder. The web or sheet of substrate to be printed is entrained between the impression cylinder and the plate cylinder. The web or sheet of substrate to be printed is entrained between the impression cylinder and the plate cylinder. With a "hard" plate, the impression cylinder must be relatively less stiff, in order to avoid damage due to mechanical interference, and/or to improve the evenness of ink transfer from the printing plate to the substrate to be printed.
The second letterpress category utilizes a printing plate (commonly referred to in the industry as a "soft" plate) whose stiffness is relatively lower; i.e. the raised areas which are to be inked and then transfer the ink to the substrate are relatively less stiff with respect to the relatively more stiff form cylinder(s) and relatively more stiff impression roll (frequently steel).
The term "flexographic" is often utilized to refer to the letterpress system in which a less stiff plate is used, with the other two rollers being relatively more stiff.
In the raised plate printing method the plates are normally made with as uniform a total thickness as is possible.
The printing industry generally recognizes certain inherent problems relating to the raised plate printing method. One of these problems relates to the different contact pressure requirements between the printing surface of the plate and the substrate, depending upon the area of coverage of the ink. It is known that the degree of contact pressure between a plate's surface and the substrate is preferably less for the less covered areas, and more for the more covered areas. When the area less covered includes tiny dots due to the four-colour separation process, it is found generally that the contact pressure necessary to properly print solid-ink areas is too high to allow correct printing of the dotted areas, because excessive contact pressure in the latter tends to expel ink from the space between the paper and the raised dot on the plate, thus forming a ring or doughnut of solid ink around a central zone of inadequate ink coverage. However, if the contact pressure between the plate and the impression roll is reduced to a level which allows a good printing of the dot, it is found that areas of solid ink are inadequately printed, i.e. the ink is not fully and/or properly transferred to the substrate.
It is known to provide, for use with a printing plate, a "make ready" plate which corresponds to the plate in the sense that the "make ready" plate has an increased thickness in the regions corresponding to the more solid ink printing, and a gradually decreasing thickness in proportion to the degree of ink coverage in other regions of the plate. Areas of low ink coverage will include locations where fine copy appears. The "make ready" is positioned under the plate with corresponding areas matched, so that all solid regions will tend to be urged more strongly against the substrate than are the areas which are only partially ink covered. It is understood that this process works to some extent, but not fully. It involves considerable extra expense to fabricate the "make ready" sheet, and it complicates the process of affixing the plate to the plate cylinder. Relative to the affixing of the plate to the plate cylinder, where a plate of relatively low stiffness is utilized without the "make ready plate", it is typical in the industry to use a sheet of two-sided adhesive tape between the plate and the cylinder. Such tape may be very compliant (referred to in the trade as "cushion tape"), incorporating a layer of open or closed cell foam which is usually very low in stiffness. It is also known to use relatively stiff or non-compliant tape. It has been found that, when a low-stiffness tape is used to secure the plate to the plate cylinder, the plate-to-substrate contact pressure drops off too greatly in the locations of high ink coverage (area-wise), while the contact pressure between plate and substrate in the locations of relatively low ink coverage (area-wise) tends to allow more acceptable printing as the dots become smaller. The low-ink coverage areas are referred to as the highlight areas of the four colour printing process. Conversely, when a stiff tape is used, the dot areas extrude ink outwardly to a larger diameter than originally intended, and the locations of heavy ink coverage (area-wise) usually print relatively properly.